1. Technical Field
The present invention relates to a hysteretic switching regulator and a control method thereof, and more particularly, to a variable-output, hysteretic switching voltage regulator having an operating frequency insensitive to variations in input and output voltages and a control method for use in such a voltage regulator.
2. Discussion of the Background
With currently increasing concern for environmental and ecological issues, there is a growing need for power-efficient electronic equipment. Reducing power consumption is doubly important for battery-powered electronic devices, such as mobile phones and digital cameras, since it not only contributes to preserving the environment but also extends the lifetime of batteries commonly used in such hand-held portable electronics.
On the other hand, most modern electronic products process large amounts of data as they become capable of recording, processing, and displaying still pictures as well as videos with sounds. The sophistication of multimedia electronic devices requires faster processing capabilities, and thus involves higher-performance and higher-speed hardware components such as central processing units (CPUs) operating with clock signals of ever-increasing frequencies. However, using a faster clock signal often results in a larger amount of power required to operate an electronic circuit, since power consumption in individual circuit components increases proportionally to the clock rate at which they are operated. Moreover, generating high frequency clock pulses itself requires a high voltage power supply.
To simultaneously satisfy both these energy-saving and fast-processing requirements, current battery-powered electronic devices have been developed featuring two modes of operation: a low-power mode in which they operate with a relatively slow clock signal and a relatively low power supply voltage to reduce power consumption, and a high-performance mode in which they use a relatively fast clock signal and a relatively high power supply voltage to enable fast processing of vast amounts of data. Switching the operation mode depending on the task being executed (e.g., using high-performance mode only when processing video data) minimizes the overall power consumption in these dual-mode electronics.
Not surprisingly, such dual-mode capability requires an appropriate power supply circuit for proper operation. To date, inductor-based, non-isolated switching regulators are widely used in battery-powered electronic devices due to their compact size and high energy efficiency, which uses a switching element or transistor alternately turning on and off current flow therethrough to temporarily charge and discharge an inductor to generate a constant output voltage.
One particular type of switching regulator is a hysteretic regulator that incorporates a hysteresis comparator to compare an output feedback voltage against a reference voltage with a certain amount of hysteresis. Hysteretic switching regulators are suitable for applications in dual-mode electronic devices such as those described above, where fast responsiveness to transient changes in load current, adjusting an output voltage according to a voltage-setting signal, and reducing energy consumption during low-power mode operation are highly required.
Conventionally, a hysteretic voltage regulator uses a resistor connected in series with an inductor to detect a current flowing through the inductor for control of an output voltage through a feedback control loop. A drawback encountered by such a conventional design is that using a current sensing resistor affects current-voltage characteristics and results in significant power dissipation, leading to low power conversion efficiency of the voltage regulator. Other drawbacks include a relatively large amount of ripple voltage superimposed on the output voltage, and variations in the operating frequency caused by variations in input and output voltages.
Several methods have been proposed to address drawbacks of conventional hysteretic switching regulators. For example, one approach is to use a low-pass filter to feed an alternating current component to a feedback control loop, which eliminates the need for a current sensing resistor connected in series with an inductor. FIG. 1 is a circuit diagram illustrating a hysteretic switching regulator 100 having such a low-pass filter instead of a current sensing resistor in a feedback control loop.
As shown in FIG. 1, the switching regulator 100 includes an output stage formed of a switch 5101 and a rectifier D101 connected in series between an input terminal IN and ground GND, and an inductor L101 having one end connected to a switch node Lx between the switch 5101 and the rectifier D101 and another end connected to an output capacitor Co and an output terminal OUT. Also included are a pair of resistors R101 and R102 connected in series between the output terminal OUT and ground GND, as well as a hysteresis comparator 121 with an inverting input connected to a feedback node Lfb between the resistors R101 and R102, a non-inverting input connected to a reference voltage generator, and an output connected to a control terminal of the switch S101, which together form a feedback control loop to control operation of the regulator output stage.
During operation, the switch 101 alternately switches on and off current flow therethrough according to a control signal HYSo to convert an input voltage Vin supplied to the input terminal IN to generate an output voltage Vout at the output terminal OUT for output to a load circuit LOAD. The output voltage Vout is monitored by the resistors R101 and R102 which divides the voltage Vout to generate a feedback voltage Vfb at the feedback node Lfb for input to the hysteresis comparator 121. The comparator 121 compares the incoming signal Vfb against a reference voltage Vref to generate the control signal HYSo, in which it exhibits a certain amount of hysteresis voltage Vhys for switching the output signal HYSo between high and low levels.
Specifically, when the output voltage Vout decreases to cause the feedback voltage Vfb to fall below the reference voltage Vref, the output of the hysteresis comparator 121 becomes high to turn on the switch 101. This results in a current flowing from the input terminal IN to the load circuit LOAD through the inductor L101, while charging the inductor L101 as well as the capacitor Co to gradually increase the output voltage Vout.
Then, as the output voltage Vout increases sufficiently to cause the feedback voltage Vfb to exceed the reference voltage Vref plus the hysteresis voltage Vhys, the output of the hysteresis comparator 121 becomes low to turn off the switch 101. This results in the voltage at the switch node Lx falling below the ground voltage GND due to a counter-electromotive force developed across the inductor L101, which in turn causes a current to flow from the ground GND to the inductor L101 through the rectifier D101, while discharging the inductor L101 and the capacitor Co to gradually decrease the output voltage Vout.
When the output voltage Vout sufficiently decreases to again cause the feedback voltage Vfb to fall below the reference voltage Vref, the above cycle repeats to continue voltage regulation.
Thus, during operation of the hysteretic switching regulator 100, the feedback voltage Vfb oscillates between its lower threshold Vref and upper threshold Vref+Vhys. In general, an oscillating feedback voltage in a hysteretic regulator creates a ripple voltage equal to vhys*(r101+r102)/r102 superimposed on an output voltage, where vhys is a hysteresis voltage, and r101 and r102 are resistances of voltage divider resistors generating the feedback voltage. Such a ripple voltage can be considerable and can adversely affect proper operation of a load circuit connected to an output terminal of the hysteretic regulator.
To reduce the voltage ripple at the output terminal OUT, the hysteretic regulator 100 additionally incorporates a pair of feedback resistors Rfa and Rfb connected in series between the switch node Lx and ground GND, and a capacitor CR connected in parallel with the resistor Rfb, which together form a low-pass filter connected to the feedback node Lfb through a series circuit composed of a unity gain buffer 122 and a resistor R103.
During voltage regulation, the low-pass filter converts a rectangular waveform generated at the switch node Lx into a triangular waveform, which is added to a waveform proportional to the output voltage Vout to generate the feedback voltage Vfb at the feedback node Lfb. Operating parameters of the hysteretic regulator 100 are selected so that the feedback voltage Vfb has its alternating current (AC) component dependent on variations in the voltage at the switch node Lx rather than the output voltage Vout.
According to this method, feeding the voltage at the switch node Lx back to the comparator input through the low-pass filter and the buffer 122 allows the hysteresis comparator 121 to switch its output HYSo to turn on/off the switch S101 before the output voltage Vout significantly varies from its nominal value, resulting in a relatively small amount of ripple voltage superimposed on the output voltage Vout.
However, the above-described switching regulator 100 still has several drawbacks. For example, the operating frequency of the switch S101 can vary significantly as the input voltage Vin varies. This is because the input voltage Vin determines the voltage at the switch node Lx, which in turn determines the AC component of the feedback voltage Vfb supplied to the hysteresis comparator 121 generating the switch control signal HYSo.
Another drawback is high costs involved in using the unity gain buffer 122 in the switching regulator 100, which is designed to operate with relatively high switching frequencies to meet today's requirements for high efficiency and compact power supply circuitry. That is, when operated at a high switching frequency, the analog buffer 122 consumes large amounts of power and requires a relatively large circuit area for proper phase compensation, which can negate some advantages of using a switching voltage regulator.